This is a proposal for the continuation of the study of models, or explanations of changes in sensitivity in normal and abnormal visual systems. A number of psychophysical techniques are employed including a measure of suprathreshold activity, the probe-flash technique. Using these psychophysical techniques, we propose to test explanations for changes in sensitivity due to adaptation, retinal locus, and retinal disease. The laboratory studies include the examination of dynamic and steady state adaptation of the blue/yellow system. This system offers a number of advantages for answering basic questions. In the clinic, we have found that the probe-flash paradigm can be used to test alternative explanations for retinal disease. This technique will be used in conjunction with other standard psychophysical techniques to test alternative explanations for a variety of heredoretinal degenerative diseases, including retinitis pigmentosa. Another interest is the vulnerability of the blue cone system to retinal disease. There are a number of hypotheses for this vulnerability and many of these can be tested psychophysically. Finally, we intend to obtain a better understanding of the relative involvement of the rod and different cone mechanisms in retinitis pigmentosa and in progressive cone dystrophy.